In apple documentation you could find info about auto-renewable subs:
every renewal of the auto-renewable subscription new transaction would be generated and send to your application. In theory, MKStoreKit must take care about all this stuff, and isSubscriptionActive should work.

Also, you could add button for restoring previous purchases. So, in your case, when user press buy button before calling -isSubscriptionActive call -restorePreviousTransactions, or you can add this button, so user can restore previous transaction^ if he already has bought subscription.

There is an open issue on the MKStoreKit GitHub that might be related to what you are experiencing. The people in that thread claim that after the app is initialized MKStoreKit always returns NO for isSubscriptionActive. It seems they were able to successfully use a previous release of MKStoreKit, though. You might want to try that.